Receiver desensitizing circuit



Feb. 13,

VROQMAN RECEIVER DESENSITIZING CIRCUIT Filed Oct. 26, 1942 400/0. FREQ. STAGES/ l T BAA/SHIT 7' E H Inventor: Donald F. Vr'ooman,

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Patented Feb. 13, 1945 RECEIVER DESENSITIZING cmcnrr Donald F. Vrooman, Scotia, N." Y., assignor to I, General Electric Company, a corporation of New York Application October 26, 1942, Serial No. 463,320 3 Claims. (Cl. 250 13) My invention relates to short-Wave radio circuits of the type used in two-way communication systems and it has for an object to provide an improved circuit for desensitizing the receiver pf such a system 'from the high voltages ofthe transmitter.

In signalling systems where a receiver employs the same antenna as the transmitter, necessary precaution must be taken to keep the high intensity transmitter radio frequency voltages out of the receiver. In such systems, it is customary to short-circuit the receiver antenna connection during a transmitting period. In mobile two-way signalling systems, the transmitter and the receiver are usually located closely together in a unitary structure and the proximity of thereceiver to the transmitter permits suflicient signal from the transmitter to be picked up by the receiver to overload the receiver equipment even though there is no antenna" connected to it. Particularly in systems where a plurality of loud speakers or head sets are connected to the equipment and a portion of the transmitted signal is supplied to these speakers or other receivingdevices in order that the transmitted, as well as received, messages may be communicated to allv of these receivers, it is increasingly diflicult, because a portion of the receiving equipment is utilized for such communication, to isolate effectively the receiving equipment from the transmitter voltages. It is customary in' desensitizing circuits for such receivers to open the voltage supply or to shortcircuit the high potential side of a signalling circuit. Th'e first of thesemethods causes severe clicks in the output of the receiver which are particularly annoying inthis type of communication circuit. On the other hand, short-circuiting the high potential side of a radio frequency circuit causes noise pick-up and sometimes regeneration of oscillation due to added switch connections, producing objectionable noise in the loud speakers and head setsj Short-circuiting the the received messages are supplied to a plurality of utilization means, such as loud speaker 4 and head sets 5. The loud speaker and head set 5 are directly connected to the output circuit of the receiver 2, which in turn is connected by means of conductor 6 to the transmitter l to receive a .sidetone signal from the transmitter during sending periods for the purpose of auditing transmitted messages.

The receiver 2 preferably is of the superheterodyne type and comprises the radio frequency and intermediate frequency stages [0, a final intermediate frequency stage II and the audio frequency stages l2, the output of the audio stages being supplied directly to the speaker 4 and the head sets 5. Preferably, both the transmitter l and the receiver 2 are completely shielded andare connected only by theconductor l3 supplying signals from antenna 3- to the receiver l0,

conductor 6 supplying the sidetone signal from means for short-circuiting the connection to the 1 receiver l0. Such a shOrt-circuiting means is indicated in my apparatus by the switch'l l which in one position connects the conductor '13 tothe antenna 3 and in its other position connects the conductor l3 directly to ground, hort-circuiting the input'of the radio frequency receiver I 0. Be-I cause of the proximity of the receiver 2 to the transmitter l and the antenna 3, complete shield ing of the receiver from radio frequency voltages is difficult to obtain. Minute radiofrequency voltages introduced into the radio frequency stages through inductive linking of the conductor I3 are amplified in the radio frequency and intermediate frequency stages so that they reach objectionable magnitudes in the output stages of the receiver. Thus, in the presence of an antenna 3 radiating signals that have intensities of as high as 10,000 volts, it has been found that it is riods by means of the switch IS in the cathode circuit of the final intermediate frequency amplifier ll. As shown in the drawing, the circuit of the cathode l6 comprises the resistor 1! connected to ground through conductor 23' and switch and the by-pass capacitor [8 conplaying a, plurality of loud speakers and head nected across resistor l1 and switch15. Prefer-J ably, the position of the switch I5 is controlled, by relay l9 operated by the switch 'located th transm tt r v and used-riot ener iz ng the r .teri Volta eior the operatio o relay 19 may be supplied from any suitable source, such as the batter .21. Lik wis r e a l the r lay l9 controls simultane uslyth op at on of switch M 01 short-ci cuiti h n u ore ceiver 2, switch l5 for isolating the radio frequency and audio frequency circuits of receiver 2, and switch 22 for supplying a sidetone signal .irom transm tter I to the output circuits f r ceiver 2,

The desensitizing circuit described above completely isolates the radio frequency and audio frequency circuits of receiver 2 by opening the cathode circuit of the last intermediate frequency amplifying tube between the bias resistor l1 and ground. This is a circuit for unidirectional currents of very small value and of low potential.

When the switch I5 is open, anode current cannot flow throughamplifier 'II and signals, consequently, cannot be transferred to .theaudio stages 3| 2. Asa result, the receiver output circuit is made completely insensitive to transmitter signals impressed on the preceding stages of the It is desirable that interruption of the 1 translating circuit through receiver 2 be made in i the final stage of radio frequency circuits.

ecei e Should the cathode circuit. of some previous stage be interrupted, due to the presence of the intense signals of the transmitter l and their linkage with the tuned circuits of subsequent stagesof radio irequency mplification, a certain amount .ojtransmitter signal is picked up in the subseq ent s ag s and, after amplific i n, intro d into the detector a d udio tases'll.

Further aco dance :with m invent capa tor i8 is made suifieiently large o p nt quick variation of the voltage of cathode IS with respect to ground when switch I5 is openedand closed. Thus, the time constant of the circuit comprising resistorl I and capacitor I8 is suffioient y large t no n ticeabl d s urban s are n oduc d to the c rcui f re eive y peration o sw t h 5, manner, all obie t'onable click andisimi r dis u ances are eli -na ed, It is desira l m eo er, that oaththus I l8 permanently connected to ground. ,If cathode IB is isolated from ground during periods of operation of the transmitter I, the cathode floats at an indeterminate radio frequency potential and, in the presence of strong signals produced by the preceding stages of the receiver, amplifier ll acts to transmit a certain amount of signal to the subsequent stages, introducing distortions "which interfere with the sidetone signal being supplied directly from the transmitter.

It will thus be seen that my invention provides an improved desensitizing circuit for the receiving apparatus of a two-Way signalling system which is particularly adapted to overcome the di-fliculties encountered in mobile systems emsets.

'While' I hav shown a particular embodiment of my invention, it will of course be understood that I'do not wish to be limited thereto since var ous mo ificati ns m y be made, and I S ontempiate by the appended claims to cover any su h mod ficat o as f w thin the true sp rit a d st ne of .my invention. i

What I claim as new and desire to secure by Letters Patent of the United States is: v

1. In a signalling system comprising ,a transmitter and a receiver both connected to a single antenna, ,said receiver including a final stage ode 6. b mai tain d a gr und potential or radi fr uency o t es by means of apacitor high frequency amplifier, an audio frequency circuit, and a plurality of utilization means connected to said circuit, means for connecting said utilization means to said transmitter for auditing transmitted signals, and means for desensitizing said receiver during operation of said transmitter, said desensitizing means comprising means for disconnecting said receiver from said antenna, and means for isolating said amplifier from said circuit.

2. In combination, a signalling system com.- prising a transmitter. and a receiver both con.-

' nected to a single antenna, said receiver including a'high frequency circuit, an audio frequency circuit, and a plurality of utilization means conv nectedto said audio circuit, means for connecting said utilization means to said transmitter for auditing transmitted signals, and means for desensitizing said receiver during operation of said transmitter, said desensitizing means comprising means for disconnecting said receiver from said antenna, and means for isolating said high fre- .quency circuit from said' audio frequency circuit.

3. In combination, a signalling system coma prevent the occurrence of objectionable noise. I

' DONALD VBOOMAN. i 

